The present invention was created after the use, development and testing of equipment for application in hazardous and non-hazardous atmospheres.
Accompanying and observing preliminary tests, it was noted several times that the lack or excess of grip on cable glands was responsible for failure in the equipment tests.
It was noticed then the need of something in the cable gland that could indicate to the fitter the correct grip that must be applied to the cable gland for each cable type utilized.
The cable gland that constitutes the previous technique refers to an accessory for general electric installations, allowing the installation of cables in electric equipment without changing the level and type of protection of these, for example the cable gland M20 for cables with 8-11.5 mm. Other examples are Type A2, for non-armored cables; Type A2F, identical to A2, however specific for utilization in explosion-proof equipment; Type C, cable gland used in armored cable to provide electrical continuity between the frame and the body of the cable gland, being provided with a sealing ring to the outer cable jacket; Type E1F, used in armored cables to provide electrical continuity between the frame and the body of the cable gland, being provided of two sealing rings, one for the outer jacket and one for the inner jacket of the cable (this model should be specified for utilization in explosion-proof equipment).
There are other less ordered models, with the same functional characteristic.
The cable glands can be manufactured with any material that can be used in its manufacturing, since it can respect requirements prescribed in regulations.
The most common cable gland materials are stainless steel body, brass, nickel plated brass, galvanized carbon steel and polyamide; and elastomeric sealing rings, accompanied of slip rings. Usually, the parts that need grip have hexagonal shape with rounded vertices.
The referred accessory is constituted of two basic pieces, in plug-and-socket screwing system (see FIG. 1). The first piece has at one of its ends an external thread, or body thread, which is used as identification. It relates to the range of diameters of the cables being used, and it connects to others installation elements by screwing to others installation elements, referred first piece receiving, on its another extremity, a second piece.
The most commonly used body threads are the following: Thread NPT and Thread BSP of ½″; ¾″; 1″; 1¼″; 1½″; 2″; 2¼″; 2½″; 3″; Metric thread of M16; M20; M25; M32; M40; M50; M63; M75; and, in smaller scale, Thread Pg type PG7; PG9; PG11; PG13,5; PG16; PG21; PG29; PG36; PG42; PG48.
The body thread determines the preparation of the local for the installation of the cable gland on the equipment. The inside of the base-thread comprises a hollow cylinder with internal screwing in part of its length and linear funneling on its end, starting after the end of the screwing section, until the beginning of the union thread.
The first piece receptions on its interior a sealing and tightening rubber element, with sliding washer, comprising inner sealing ring.
The second piece, screwed on the opposite extremity of the first piece, constitutes an plug element that, as it's screwed to the first piece, compresses the inner sealing ring in a way that the inner diameter of the inner sealing ring decreases and the outer increases, thus tightening and isolating the cable that go through the cable gland.
With the compression exercised by the screwing of the second piece, the outer wall of the inner sealing ring expands over the inner wall of the first piece and further the outer wall of the referred element projects by the inner conical extremity of the first piece, thus progressively restricting the passage space until completely sealing the passage which the cable passes through.
In the whole world, the electrical equipment projected to be utilized in hazardous atmospheres and in non-classified areas (exposed to the action of the environment) need to attend security requirements prescribed in regulations in order to be commercialized.
A hazardous atmosphere consists in an environment where determined proportions of gas, steam, dust or fibbers in contact with the oxygen (oxidant) and a heat source (spark from an electric circuit or heating of an equipment) can provoke an explosion. Therefore, electric equipment installed in these locations should eliminate or isolate the ignition source, preventing the simultaneous occurrence of the three components that form the fire triangle: fuel, oxygen and ignition source. This is of the biggest challenges and worries of the designers.
To prove that the equipment meet the necessary requirements to be commercialized, they pass through a battery of laboratory tests accredited by a governmental agency. If they are approved, a Conformity Certificate is emitted validating the kind and the level of protection that the equipment was projected for.
For these equipment function, is also necessary that there is a safe electric installation. According to the actual norms, there are three allowed installation methods: the American methodology (metallic conduits+explosion proof boxes+sealing units); the European (cables+cable glands); and the mixed system, that provides utilization on the installation of the American and European systems, alternately and according to the verified necessity in each part of the installation.
Among these, the European method is the most used. Its advantages are: great flexibility, economy and quick installation. Said method consists basically in installing the connecting cables on the fasteners. However, there are disadvantages: the system is completely open, subjecting the wiring to eventual damage. Therefore, this method should not be used in locations subject to mechanical damage or chemical agents (in this case is recommended utilization of the mixed system, in which the cables are protected by conduits in locals of exposition to potential mechanical damage and/or chemical). The sealing system of the European method is based in the utilization of accessories known as “cable glands”.
The equipment installations should be appropriate for each kind of environment. Then, require inspection by qualified professionals according to specified norms, to maintain the equipment's special characteristics. The cable glands play a very important role in this context, because they are installed at the entrances and the exits of electric cables of the equipment to maintain the integrity of these. Therefore, they need to attend to the prescribed security requirements and possess a conformity agreement emitted by an governmental agency attesting kind and level of protection compatible with these equipment.
Despite every worry about the security of commercialized cable glands, currently do not exist sealing parameters for cable glands that assist the fitter to reproduce the exact sealing point for every cable. The approved cable gland models use for each gauge a small range of electric cables diameters. These accessories are specified by the model of its body thread. Moreover, for each thread gauge is also related the diameter range of the electric cables that can be used.
In the current technique, the sealing is obtained by the effort put on the pressure of the cable gland by the fitter, on his own criteria. This effort can be easily misunderstood with some unseen detritus or even a small damage on the thread fillet. This generates a series of risks that range from electric cable damage by excess clamp, to explosion risk by the false feeling of clamp. Therefore, a special attention should be paid to the sealing functional system of the cable glands.
The main innovation of the present invention consists in eliminate the problem about the sealing of the current cable glands, by the application of elements that determines the effort to be put by the fitter to install the cable gland, in a way to prevent irreversible damage to the installations of industrial plants, residences, condos, etc.